Add Xinerama support for GHOST_GetMainDisplayDimensions() so X11 works as it should (previously would include all monitors).

Now the active monitor size is used on startup. 
Currently the cursor position is checked for intersection with the monitor bounds to find the active screen.

source/blender/windowmanager/intern/wm_window.c

@@ -436,18 +436,11 @@ void wm_window_add_ghostwindows(wmWindowManager *wm)
 			wm_set_apple_prefsize(wm_init_state.size_x, wm_init_state.size_y);
 		}
 #else
-		/* default size when un-maximized, unless the screen(s) are smaller */
-
-		/* clamp by these arbitrary values because currently wm_get_screensize()
-		 * will span multiple monitors and using xinerama isnt totally reliable
-		 * since we don't which monitor the window manager will put the blender
-		 * window in. */
-		wm_init_state.size_x = MIN2(1800, wm_init_state.size_x - 100);
-		wm_init_state.size_y = MIN2(980,  wm_init_state.size_y - 100);
-
+		/* note!, this isnt quite correct, active screen maybe offset 1000s if PX,
+		 * we'd need a wm_get_screensize like function that gives offset,
+		 * in practice the window manager will likely move to the correct monitor */
 		wm_init_state.start_x = 0;
 		wm_init_state.start_y = 0;
-		
 #endif
 	}